1. Field of the Invention
The present invention relates to a focal length varying device for a zoom lens, capable of varying the focal length continuously to the telephoto side or to the wide angle side.
2. Related Background Art
The conventional zoom lens of this kind has at least two lens groups in the lens barrel and is rendered capable of varying the focal length by the change in the distance between said lens groups, thereby enabling the phototaking operation at an arbitrary image magnification. In such zoom lens, the position of the focal plane, or the rear focal length, has to be constant regardless of the zooming position.
In such zoom lens, in response for example to the rotation of a rotatable tube in a predetermined direction, a first lens group in a supporting tube is axially advanced or retracted by a linear helicoid mechanism and a cam tube linked with said rotatable tube is rotated to induce a relative displacement between a circumferential cam groove and an axial cam groove of a guide tube, thereby axially moving a second lens group frontward or backward at the same time with a change in the distance from said first lens group by means of cam pins engaging with said cam grooves, whereby the phototaking operation is rendered possible at an arbitrary image magnification.
However, the above-mentioned cam tube and guide tube, if molded with a plastic material, will inevitably show parting lines, as a result of mold splitting, on the cam faces of the circumferential or axial cam groove. Such parting line constitutes a protrusion on the proper cam face, prone to stop a cam pin, whereby the distance between the first and second lens groups becomes deviated from the designed value and the focal plane no longer remains, therefore, in the constant position.